1. Field of the Invention
The present invention relates to an ultra-thin semiconductor chip according to a new ultra-thin silicon-on-insulator (SOI) substrate production method, more particularly a method and an apparatus for producing a back-illuminated solid-state image pickup device for detecting light incident from a back side of the substrate of an ultra-thin single-surface resin sealed chip size hollow package according to the new ultra-thin SOI substrate production method.
2. Description of the Related Art
For an image sensor having sensitivity with respect to an input of UV rays, soft X-rays, or an electron beam, use is being made of a back-illuminated charge coupled device (CCD) etc.
Namely, UV rays, soft X-rays, and electron beams have a large absorption coefficient, so a CCD having no polycrystalline silicon electrodes or other obstacle at the incident surface, that is, a back-illuminated CCD illuminated from the back of the CCD forming surface is preferred.
A usual silicon substrate has a thickness of 400 to 500 μm, however, so it cannot be used as is for a back-illuminated. Therefore the back of this silicon substrate is made thinner mechanically and chemically, that is, by grinding down the back, by grinding down the back and polishing the back, or by grinding down the back and chemical etching.
The technology disclosed in for example Japanese Unexamined Patent Publication (Kokai) No. 6-326293 comprises first forming a polyimide resin film on a substrate surface having a CCD back-illuminated function formed thereon, affixing substitute silicon substrate on this, grinding down the entire back of the CCD substrate, then polishing the back to a mirror surface state. Next, it calls for chemically etching the polished substrate surface to remove the altered layer near the front surface of the substrate caused due to the mechanical grinding and thereby obtain a back-illuminated light detection substrate having a desired substrate thickness.
Further, the technology disclosed in Japanese Unexamined Patent Publication (Kokai) No. 7-245386 comprises forming a silicon nitride film on the back of the substrate having the CCD back-illuminated function formed thereon, removing the silicon nitride film at the center of the back of the chip where light etc. will strike while leaving the silicon nitride film at the chip periphery in a frame state and using it as a mask at the time of silicon chemical etching, and chemically etching the back to thereby obtain a predetermined thickness of 10 to 20 μm.
Summarizing the problems to be solved by the invention, with such mechanical, chemical, or mechanical and chemical methods, variation occurs in the substrate thickness needed at the center of the back of the chip where light etc. will strike, the light reception sensitivity of the CCD sensor will become uneven, fixed pattern noise due to dark current will increase, and other problems will easily arise.
Further, a chip becomes susceptible to mechanical shock and temperature changes along with the reduction of the thickness, so breaks, becomes chipped, etc. leading to a drop in the yield and the quality. Also, the increase in the number of work steps leads to a deterioration of the productivity and consequently an unavoidable increase in costs.
Japanese Unexamined Patent Publication (Kokai) No. 2001-267542 discloses technology concerning an infrared ray sensor comprised of an SOI layer formed with a pn junction, while Japanese Unexamined Patent Publication (Kokai) No. 2000-88640 and Japanese Unexamined Patent Publication (Kokai) No. 9-166497 disclose technology concerning an infrared ray detector utilizing an SOI substrate.
Accordingly, in a back-illuminated solid-state image pickup device as well, it is also considered to utilize an SOI layer as a thin semiconductor layer for back-illuminated to eliminate the mechanical and chemical polishing. In actuality, Japanese Unexamined Patent Publication (Kokai) No. 10-209417 discloses technology concerning a solid-state image pickup device for detecting X-rays, γ-rays, and charged particles utilizing a bonded SOI substrate.
Here, the problems occurring when trying to fabricate a thin semiconductor chip and a thin back-illuminated solid-state image pickup device by utilizing the well known SOI substrate fabrication processes will be explained in detail below.
At the present, as methods for the production of an SOI substrate, the ELTRAN (epitaxial layer transfer (Canon)) method, hydrogen ion peeling method (also referred to as the “SMART CUT method” (Commissariat a l'Energie Atomique; France)), the SIMOX method, etc. are known.
The ELTRAN method disclosed in Japanese Patent No. 2608351, well known ELTRAN technical papers, etc. comprises first chemically processing a seed Si wafer surface to a sponge structure of a porous Si layer formed with innumerable fine holes having a diameter of 0.01 μm by anodic oxidation. Then, it calls for epitaxially growing a single crystalline Si layer on this porous Si layer. Further, it thermally oxidizes this single crystalline Si layer surface to form an insulating film, bonds this with a handle Si wafer, then separates the seed Si wafer at the porous layer by a water jet. Next, it removes the porous layer left on the handle Si wafer by ultra-high selective etching and finally flattens the surface by hydrogen annealing to thereby fabricate the SOI substrate.
That is, this is a bond and etch-back silicon-on-insulator (BESOI) structure obtained by a combination of epitaxial growth and surface flattening by hydrogen annealing on a porous Si layer processable by ultra-high selective etching.
The above method is characterized by separating the seed Si wafer at the porous Si layer by a water jet, but the thinner this single crystalline Si layer and the larger the wafer size, the harder the separation and the more easily problems arise in the yield and quality due to breakage, chipping, cracks, etc.
Further, the technology disclosed in Japanese Unexamined Patent Publication (Kokai) No. 11-195562 calls for forming a plurality of porous Si layers having different porosities as the above porous layer to thereby facilitate the peeling, but in this case as well, similarly, at the time of peeling by tension the porous Si layer, problems easily arise in the yield and quality due to breakage, chipping, cracks, etc.
The hydrogen ion separation method (SMARTCUT method) comprises forming a hydrogen ion implanted layer at a location of a predetermined depth from the Si wafer surface, bonding the result with an Si wafer separately thermally oxidized to form an insulating film, then heat treating the result and peeling the layers apart at the hydrogen ion implanted layer and finally flattening the surface by hydrogen annealing to thereby fabricate an SOI substrate (refer to for example Japanese Patent No. 3048201, Japanese Unexamined Patent Publication (Kokai) No. 2000-196047, Japanese Unexamined Patent Publication (Kokai) No. 2001-77044, and Japanese Unexamined Patent Publication (Kokai) No. 5-211128).
The above method is characterized by causing strain in the high hydrogen ion implanted layer by the action of the pressure in the micro bubbles of hydrogen and the action of crystal realignment and pulling apart and separating the two substrates. In this method as well, the thinner this single crystalline Si layer and the larger the wafer size, the harder the separation and easier problems arise in the yield and quality due to breakage, chipping, cracks. etc.
As explained above, when trying to fabricate a thin semiconductor chip and a thin back-illuminated solid-state image pickup device by utilizing these well known SOI substrate fabrication processes, in each case, separation becomes difficult when the wafer size becomes large and consequently there is a problem in the yield, quality, and reliability due to breakage, chipping, cracks, etc.